Thermostatic device

ABSTRACT

A thermosensitive control device comprising a rigid thermally conductive casing closed at one end and open at the other end, said casing including a substantially cylindrical enclosed housing having first and second opposed end walls and a sidewall, said first and second end walls being of greater thickness than said sidewall and said first end wall having a single central opening provided therein and an elongated tubular casing section having a cross-sectional area which is less than the crosssectional area of said housing, said casing section having open inner and outer ends, the inner end thereof being inserted into said housing through said single central opening in the first end wall thereof, said first end wall having an inner surface which angles outwardly away from the inner end of said casing section toward said sidewall and second end wall a single thermal drive element formed by a thermally sensitive, inelastic plastic rod expandable as substantially a solid at elevated temperatures below 300* F. received within said housing and casing section and abutting said second end wall, the portion of said rod within said housing being of greater diameter than the portion of said rod within said casing section whereby the outer surfaces of said rod are in contact with the inner surfaces of said housing sidewall and casing section, a drive piston received within the open outer end of said casing section, said drive piston being movable outwardly of said casing in response to the expansion of said plastic rod at elevated temperatures, and biasing means mounted upon said piston, said biasing means being operative to bias said piston toward said plastic rod to maintain said sealing means in tight sealing engagement with the inner surface of said casing section.

United States Patent [72] Inventor [54] THERMOSTATIC DEVICE 5Claims,6l)rawing Figs.

52 U.S.Cl. 236/93, 73/368, 73/3683, 236/101 51 lnt.Cl 0056 23 02, 601k 5 00 50 FieldotSearch 73/3623,

Primary Examiner-Janis R. Prince Assistant Examinerwilliam A. Henry, H

ABSTRACT: A thermosensitive control device comprising a rigid thermally conductive casing closed at one end and open at the other end, said casing including a substantially cylindrical enclosed housing having first and second opposed end walls and a sidewall, said first and second end walls being of greater thickness than said sidewall and said first end wall having a single central opening provided therein and an elongated tubular casing section having a cross-sectional area which is less than the cross-sectional area of said housing, said casing section having open inner and outer ends, the inner end thereof being inserted into said housing through said single central opening in the first end wall thereof, said first end wall having an inner surface which angles outwardly away from the inner end of said casing section toward said sidewall and second end wall a single thermal drive element formed by a thermally sensitive, inelastic plastic rod expandable as substantially a solid at elevated temperatures below 300 F. received within said housing and casing section and abutting said second end wall, the portion of said rod within said housing being of greater diameter than the portion of said rod within said casing section whereby the outer surfaces of said rod are in contact with the inner surfaces of said housing sidewall and easing section, a drive piston received within the open outer end of said casing section, said drive piston being movable outwardly of said casing in response to the expansion of said plastic rod at elevated temperatures, and biasing means mounted upon said piston, said biasing means being operative to bias said piston toward said plastic rod to maintain said sealing means in tight sealing engagement with the inner surface of said casing section.

PATENTEU JUL 6 I97! SHEET 1 or 2 PATENTEUJUL 6|97| 359L078 sum 2 UF 2 INVENTOR MAURICE FINBERG w, 3 2x5? w??? TI-IERMOSTATIC DEVICE The present application constitutes a divisional application of my earlier filed pending application, Ser. No. 313,885, filed Oct. 4, 1963.

-ing a flexible diaphragm at one end thereof. Other similar devices utilize various wax compositions either alone or in combination with powdered metallic particles.

In accordance with the present invention there is provided a novel, improved heat responsive actuator incorporating as the active element an expandable rod of plastic material. In the preferred embodiment, the driving element is in the form ofa polyethylene plastic which material is particularly well adapted for use as the active element since it remains essentially a solid during the operating range of the actuator, and

- accordingly, the need for sealing elements incident to the use of a liquid as the driving element is eliminated.

The novel actuator of the present invention does not evidence fatigue distress common to the rubber or rubberlike bellows and diaphragm devices, and additionally, it is capable of driving with a far greater force and of moving over a greater distance than previously known actuators. Since the only moving part is constituted by a sliding plastic rod and a piston within a tube the actuator may be simply and economically constructed. The plastic is not subject to aging nor to adverse environment conditions since it is at all times enclosed within a tube and any moisture which may be originally contained in the tube is forced or squeezed out upon initial actuation of the device.

' An object of the present invention is to provide an improved therrnosensitive actuator.

Another object of the present invention is to provide an actuator incorporating a thermosensitive plastic as the expandable drive element.

Yet another object of the present invention is to provide an improved heat sensitive actuator incorporating a polyethylene plastic as the expandable drive element.

Still another object of the present invention is to provide an improved heat-sensitive actuator incorporating a polyethylene rod as the expandable drive element.

A further object of the invention is to provide an improved heat-sensitive actuator employing a polyethylene plastic as the expandable drive element, which element remains essentially a solid within the operating range of the actuator.

Yet another object of the invention is to provide an improved actuator designed for use with installations of the type wherein the actuator is immersed in a liquid or exposed to a liquid environment said actuator incorporating an expandable heat sensitive plastic as the drive element and having improved sealing means constructed and arranged to prevent ingress of liquid and thereby eliminate operational deviation and inaccuracy caused thereby.

A still further object of the invention is to provide an improved heat sensitive actuator incorporating polyethylene plastic as the expandable drive element, said actuator being strong and sturdy in construction, consisting of but'relatively few parts, and not liable to get out of order even after long and continued use.

With these and other objects in view, which may be incident to my improvements, the invention consists in the parts and combinations to be hereinafter set forth and claimed, with the understanding that the several necessary elements comprising my invention, may be varied in construction, proportions and arrangements, without departing from the spirit and scope of the appended claims.

In order to make my invention more clearly understood, 1 have shown in the accompanying drawings means for carrying the same into practical effect, without limiting the improvements in their useful application to the particular constructions, which for the purpose of explanation, have been made the subject of illustration.

In the drawings:

FIG. 1 shows one embodiment of the actuator of the present invention;

FIG. 2 shows a modified form of the actuator of the present invention;

FIG. 3 shows a further modification of the actuator of the present invention;

FIG. 4 illustrates the position of the actuator of FIG. 3 at elevated temperatures.

FIG. 5 shows the actuator of FIG. 2 mounted as an immersed valve control unit; and

FIG. 6 shows the actuator of FIG. 3 mounted as an immersed valve control unit.

Referring to the drawings and more particularly to FIG. 1, the novel actuator of the present invention, indicated generally by numeral 10, comprises a tube or casing '12 formed from a suitable material adapted to house a plastic rod 14 snugly received therein. One end of the casing may be closed by an integral cap 16 or by other suitable closure means, with one end of the plastic rod engaging the end cap, and the opposite end 18 of the rod engaging the adjacent end of a push rod 20 slidably supported within the open end portion of the casing 12. The push rod 20 is provided with an annular flange 22 which acts as a stop and limits movement of the rod inwardly of the casing.

Secured to the outer surface of the casing 12, as by welding 24, is an adjusting screw panel 32 for the actuator. By rotating the screw 28 the actuator may be advanced or retracted with respect to the panel 32, and thereby change the zero setting thereof. While adjusting means are shown for varying the setting of the actuator, it will be understood, that if desired, the actuator may be preset, thus eliminating the adjusting means.

The outer end portion 34 of the push rod 20 passes through an aperture 36 in spring retainer 40 which may constitute a portion of a suitable housing for the actuator and may be physically connected to support panel 32. A suitable compressing spring 42 is interposed between the retainer 40 and the annular flange 22 of the push rod 20, whereby the push rod is biased against the plastic rod 14 during expansion and contraction thereof.

When the actuator 10 of FIG. I is subjected to increased temperatures the plastic rod 14 expands thus moving the push rod 20 to the right, as viewed in FIG. 1, so that the end portion 24 of the rod extends further beyond member 40, in order to actuate a switch, operate a valve, or perform some other useful drive function. The polyethylene rod employed as the drive element becomes plastic at approximately 194 F. and has a useful range of from I00300 F. The rod remains extremely viscous and essentially a solid within the operating range, and does not leak out between the piston or push rod 20 and the inner wall of the casing 12. In one embodiment of an actuator constructed in accordance with FIG. 1, a 10-inch rod of polyethylene received in a 12 -inch long casing expanded upon heating to a length of 12 inches. The polyethylene rod was three-sixteenths inch in diameter and had a minimum opposing spring force of 20 ft.-pounds.

The actuator arrangement of FIG. 1 is designed for use in connection with dry" installations that is to say, installations of the character wherein the actuator is not immersed in a liquid or otherwise exposed to a liquid environment. It will be appreciated that in many installations the actuators are immersed in a fluid which may be under pressure, and accordingly, means must be provided to assure their accurate and efficient operation when used with so-called "wet" installations.

Concerning the need for means to assure accurate operation of an actuator when immersed in a liquid, and particularly in a liquid under pressure, it has been found that an actuator constructed in accordance with FlG. l for example, when used with a wet installation is subject to deviation and inaccuracy caused by the ingress of liquid between the inner wall of the housing 12 (FIG. 1) and the outer surface of the polyethylene rod 14, which liquid travels inwardly from the open end 12 of the housing into contact with the end 14 of the rod, due to a combination of forces. One of the forces is the positive pressure of the liquid which drives it inwardly of the tube, and another is the result of the negative pressure developed between the end cap 16 and the inner end 14' of the rod, when the rod expands.

After considerable research and experimentation l have found that deviation and inaccuracy in the operation of the ac tuator when used with so-called wet installations, as explained hereinabove, can be eliminated by utilizing the reactive components of the forces developed by the plastic rod and the piston or push rod, to radially expand a sealing member into tight sealing engagement with the inner surface of the actuator housing. In this connection, it will be appreciated that the magnitude of the forces exerted on the sealing member is a function of the resistance to movement of the piston, and accordingly, the sealing effect of the sealing member is directly proportional to the movement or thrust of the piston.

I have also, found that under certain conditions of use, depending upon the nature of the fluid environment in which the actuator is submerged there is a tendency for foreign material carried by the liquid for example, relatively fine particles of lime, sand, metallic dust or other particulate material, to travel inwardly of the actuator housing between the piston and the inner surface of the housing into contact with the sealing member and eventually into contact with the plastic rod, thus causing scoring of the housing, and abrasion of the piston and the sealing member. The combined effect of the abrasion arid scoring of the aforementioned components is manifested by a gradual deterioration of the scaling function of the sealing member, whereby operational deviation and inaccuracy occur, by reason of ingress of the liquid into the housing and in contact with the plastic rod.

Referring to FIG. 2, and FIG. 5, there is shown a modified embodiment of the actuator of the present invention designed for use with wet" installations that is to say, installations of the type wherein the actuator is submerged in aliquid or exposed to a liquid environment which may be under pressure. in this embodiment of the invention the actuator comprises a rigid casing or tube 44 formed from hard copper. Monel metal, stainless steel or other suitable material adapted to house an expandable heat sensitive rod 45. One end of the easing may be closed by means of a sealing plug 46 formed from brass or other suitable material, the plastic rod having one end engaging the plug, the opposite end 18' of the rod engaging the adjacent end of a sealing member 47 formed from silicone rubber, Viton, or other elastomeric material, interposed between the outer end of the plastic rod and the inner end of a piston or push rod 48. It will be understood that the sealing member may be formed from any suitable natural or synthetic elastomeric material such a natural rubber, butadiene-styrene copolymers, polyisobutylene, chloroprene, copolymer blends of styrene-acrylonitrile and butadiene-acrylonitrile, which materials may be employed as replacement elastomers for the silicone rubber.

The end portion 48 of the piston or push rod 48 (FIG. 2) is formed with an annular groove 49 having a resilient O-ring 4,9 seated therein, said ring providing a tight seal between the piston and the housing, whereby to prevent foreign material such as oil, particles of sand, metallic dust, or other particulate material from coming into contact with the sealing plug and/or the plastic rod. The driving element 45 of the actuator is preferably formed from polyethlene, however, it will be appreciated that other expandable heat sensitive plastics for ex ample, various combinations of polyethylene and suitable waxes may be employed.

In the embodiment of FIG. 2, and FIG. 5 assuming that the actuator is operatively associated with a use device for example, a valve 57, and is subjected to elevated temperatures, the plastic rod 45 expands, thus moving the piston 48 to actuate the valve, or perform some other useful drive function. As the piston moves outwardly against the resistance of the operative component of the device with which the actuator is associated, the sealing member is radially expanded into tight sealing engagement with the inner surface of the tube, due to the compressive action of the forces developed between the outer end 18' of the plastic rod, and the inner end of the piston.

This improved sealing arrangement is an important feature of the present invention, since it provides a positive seal against ingress of liquid into the tube and into contact with the plastic rod in the event that the actuator is immersed in a liquid or subjected to a liquid environment which may be under pressure.

Referring to FIG. 3 and FIG. 6 there is shown another embodiment of the actuator of the present invention comprising a cylindrical shaped housing 50 having an apertured end wall 52, and a plug 54 threadably received in its opposite end, which plug preferably keyed as at 56 to provide for adjustment within the threaded portion 58 of the housing. Extending through the apertured end wall 52 of the housing is a rigid tubular casing 60 formed from hand copper or other suitable material which casing, in conjunction with housing 50, completely surrounds a heat sensitive expandable plastic rod 62 comprising an enlarged end portion 64 within the housing 50, and a portion 66 of reduced diameter snugly received in the casing 60. An annular adapter 68, positioned within the housing is secured to the housing and to the casing by brazing or other suitable means. The rear surface 70 of the adapter 68 is preferably inclined at an angle of approximately 60 with respect to the longitudinal axis of the plastic rod, for utilizing capillary attraction to achieve a greater movement of the plastic along the casing and thereby move the piston a greater distance.

Secured to the casing 60, as by welding 74, is an adjusting screw 72 having threads 76 adapted for threaded engagement with a fixed support 78 for the actuator. By-rotating the screw the actuator may be advanced or retracted with respect to the support, and thereby vary its zero setting. The end portion 82 of the push rod extends through an aperture 84 in a member 86 which may, in conjunction with the support 78, constitute a housing or framework to support the actuator in operative position with respect to the use device such as the valve 57 with which it is associated. A suitable compression spring 88 surrounding the outer end portion of the push rod 80 has one abutting the member 86, the opposite end of the spring bearing against a stop member 90 carried by the push rod whereby the push rod is biased in the direction of the plastic rod whereby to return the same to its initial unexpanded condition, and to effect tight sealing engagement between suitable means and the interior of casing 60, will be described more fully hereafter.

Reference again being made to FIG. 3 it will be noted that the left end 92 of the push rod is formed with an annular groove 94 to received a resilient O-ring seal 96 to seal the push rod against the casing 60. The O-ring may be formed from Buna-N-rubber or from silicone rubber, or other suitable elastomeric material, said O-ring functioning to exclude foreign materials such as dirt, dust, oil and other particulate material from coming into contact with the plastic rod. A compressible disc or plug 98 of silicone rubber of other suitable material is interposed between the outer end of the plastic rod and the inner end of the push rod, as shown, the disc being radially expanded into tight sealing engagement with the interior of the housing 60 due to the compressive action of the forces between the end of the plastic rod and the end of the piston, as explained in connection with the description of the embodiment of the actuator shown in FIG. 2. The primary function of the sealing member is to prevent foreign material from entering the casing and coming into contact with the plastic rod, and accordingly, it should be of such dimensions to radially expand into tight sealingengagement with the interior of the housing.

Referring to H6. 4, the actuator of FlG. 3 is shown with the plastic rod in expanded position. As will be seen, when the rod is subjected to elevated temperatures it expands along the easing 60 and forces the piston or push rod 80 a substantial distance to the right so that the end portion 82 thereof projects a greater distance through the aperture 84, which movement compresses the spring and increases its restoring force.

In order to insure uniform operation of the actuator, that is to say, insure that actuation of the device always occurs at the temperature for which it is preset, it has been found necessary to provide means for returning the plastic rod, upon cooling, to its normal unexpanded condition. The need for such return means is due to the fact that the plastic does not consistently contract to the position it occupied before being heated, and therefore the next actuation of the device does not occur at precisely the same temperatureas the previous one.

When using the actuator arrangement of FIG. 2 with use devices such as valve 57 for controlling the flow of liquids, such valves conventionally being biased into normally open or normally closed position, the biasing means 61 for the valve may be utilized to return the plastic rod to its initial unexpanded condition. In the event that the use device does not incorporate means for biasing one of its components into selected, desired position, a return spring or other biasing means, not shown may be employed.

It will be noted that the biasing means for returning the plastic rod to its initial unexpanded condition performs two functions; it not only returns the rod to its initial condition, but also maintains the sealing member interposed between the rod and the piston in sealing contact with the interior actuator casing or housing at all times thereby preventing ingress of foreign materials interiorly of the housing and into contact with the plastic.

It is apparent from the above that the present invention provides a novel thennosensitive actuator substantially improving o'ver known devices in terms of high accuracy, reliability, the generation of a powerful positioning force, and simplicity of construction. The expandable actuator of the present invention generates a greater force and moves a substantially greater distance than conventional devices. The plastic is not subject to aging or adverse environmental effects since it is enclosed in a tube and moisture may not gain access to it.

If desired, the plastic rod in any of the embodiments may be coated with a suitable lubricant such as silicone grease to facilitate the sliding movement of the plastic within the tube and to speed the contraction of the plastic when cooling.

The plastic selected for use in the present invention must have a high coefficient of expansion with corresponding low density relative to metallic solids within the desired range of temperature. Furthermore, the material must re remain essentially an expandable solid throughout the same temperature range. The term solid is used broadly since some polyethylenes can have viscosities in the range of 4,000 C.P.S. (centerpoise well above the theoretical melting point of the particular polyethylene or the point at which the crystal lattice breaks down, and yet still be effective for the present invention.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

lclaim:

l. A thennosensitive control device comprising a rigid thermally conductive casing closed at one end and open at the other end, said casing including a substantially cylindrical enclosed housing having first and second opposed endwalls and a sidewall, said first and second endwalls being of greater thickness than said sidewall and said first endwall having a single central opening provided therein and an elongated tubular casing section having a cross-sectional area which is less than the cross-sectional area of said housing, said casing section having open inner and outer ends, the inner end thereof being inserted into said housing through said single central opening in the first endwall thereof, said first endwall having an inner surface which angles outwardly away from the inner end of said casing section toward said sidewall and second endwall, a single thermal drive element formed by a thermally sensitive, inelastic plastic rod expandable as substantially a solid at elevated temperatures below 300 F. received within said housing and casing section and abutting said second endwall the portion of said rod within said housing being of greater diameter than the portion of said rod within said casing section whereby the outer surfaces of said rod are in contact with the inner surfaces of said housing sidewall and section, a drive piston received within the open outer end of said casing section, said drive piston being movable outwardly of said casing in response to the expansion of said plastic rod at elevated temperatures, and biasing means mounted upon said piston, said biasing means being operative to bias said piston toward said plastic rod to maintain said sealing means in tight sealing engagement with the inner surface of said casing section.

2. The thermosensitive control device of claim 1 wherein said piston is provided with piston sealing means extending between the piston and the inner surface of said casing section and said second housing endwall to alter the position of said plastic rod relative to said piston.

3. A thennosensitive control device comprising a rigid thermally conductive casing closed at one end and open at the other end, said casing including an enclosed housing having sidewalls and a pair of endwalls one of said endwalls fonning the closed end of said casing while the second end wall includes a single opening provided therein, and an elongated casing section having a cross-sectional area which is less than the cross-sectional area of said enclosed housing, said casing section having open inner and outer ends, the inner end thereof being received in said single opening in the second end wall of said housing, a single thermally sensitive drive member formed completely of inelastic plastic material expandable as substantially a solid at elevated temperatures received within said casing and abutting the closed end thereof, said thermally sensitive drive member substantially filling said housing and extending from said housing into said casing section, and a drive piston received within the open outer end of said casing section, said drive piston being movable outwardly of the easing section in response to the expansion of said drive member at elevated temperatures.

4. The thermosensitive control device of claim 3 wherein the end wall of said housing which receives the inner end of said casing section includes an inner surface which angles outwardly and downwardly from the inner end of said casing section.

5. ln combination with a liquid control valve having a valve closure member and biasing means to bias said closure member into a normally closed position, a thermosensitive control device mounted to actuate said valve and submerged in liquid entering the valve to be responsive to the temperature of such liquid, said thermosensitive control device comprising a rigid heat conductive casing closed at one end and open at the opposite end, said casing including a substantially cylindrical enclosed housing having first and second opposed endwall and a sidewall, said first endwall having a single opening provided therein'and an elongated tubular casing section having a cross-sectional area which is less than the cross-sectional area of said housing, said casing section having open inner and outer ends, the inner end thereof being received in said single opening in said first end wall, said first endwall hav ing an inner surface which angles outwardly away from the inner end of said casing section toward said sidewall and second endwall of the housing, an elongated thermally sensitive member formed solely of inelastic plastic material which is expandable as substantially a solid with substantially no change in viscosity at temperatures below 300 F. received within said casing and abutting said closed end thereof, said thermally sensitive member filling said housing and extending into said casing section, the portion of said thermally sensitive member within said housing being of greater cross-sectional area than the portion of said thermally sensitive member within said casing section whereby the outer surfaces of said thermally sensitive member are in contact with the inner surfaces of said casing section and housing sidewall, a piston received within the open outer end of said casing section, said 

1. A thermosensitive control device comprising a rigid thermally conductive casing closed at one end and open at the other end, said casing including a substantially cylindrical enclosed housing having first and second opposed endwalls and a sidewall, said first and second endwalls being of greater thickness than said sidewall and said first endwall having a single central opening provided therein and an elongated tubular casing section having a cross-sectional area which is less than the crosssectional area of said housing, said casing section having open inner and outer ends, the inner end thereof being inserted into said housing through said single central opening in the first endwall thereof, said first endwall having an inner surface which angles outwardly away from the inner end of said casing section toward said sidewall and second endwall, a single thermal drive element formed by a thermally sensitive, inelastic plastic rod expandable as subStantially a solid at elevated temperatures below 300* F. received within said housing and casing section and abutting said second endwall the portion of said rod within said housing being of greater diameter than the portion of said rod within said casing section whereby the outer surfaces of said rod are in contact with the inner surfaces of said housing sidewall and section, a drive piston received within the open outer end of said casing section, said drive piston being movable outwardly of said casing in response to the expansion of said plastic rod at elevated temperatures, and biasing means mounted upon said piston, said biasing means being operative to bias said piston toward said plastic rod to maintain said sealing means in tight sealing engagement with the inner surface of said casing section.
 2. The thermosensitive control device of claim 1 wherein said piston is provided with piston sealing means extending between the piston and the inner surface of said casing section and said second housing endwall to alter the position of said plastic rod relative to said piston.
 3. A thermosensitive control device comprising a rigid thermally conductive casing closed at one end and open at the other end, said casing including an enclosed housing having sidewalls and a pair of endwalls one of said endwalls forming the closed end of said casing while the second end wall includes a single opening provided therein, and an elongated casing section having a cross-sectional area which is less than the cross-sectional area of said enclosed housing, said casing section having open inner and outer ends, the inner end thereof being received in said single opening in the second end wall of said housing, a single thermally sensitive drive member formed completely of inelastic plastic material expandable as substantially a solid at elevated temperatures received within said casing and abutting the closed end thereof, said thermally sensitive drive member substantially filling said housing and extending from said housing into said casing section, and a drive piston received within the open outer end of said casing section, said drive piston being movable outwardly of the casing section in response to the expansion of said drive member at elevated temperatures.
 4. The thermosensitive control device of claim 3 wherein the end wall of said housing which receives the inner end of said casing section includes an inner surface which angles outwardly and downwardly from the inner end of said casing section.
 5. In combination with a liquid control valve having a valve closure member and biasing means to bias said closure member into a normally closed position, a thermosensitive control device mounted to actuate said valve and submerged in liquid entering the valve to be responsive to the temperature of such liquid, said thermosensitive control device comprising a rigid heat conductive casing closed at one end and open at the opposite end, said casing including a substantially cylindrical enclosed housing having first and second opposed endwall and a sidewall, said first endwall having a single opening provided therein and an elongated tubular casing section having a cross-sectional area which is less than the cross-sectional area of said housing, said casing section having open inner and outer ends, the inner end thereof being received in said single opening in said first end wall, said first endwall having an inner surface which angles outwardly away from the inner end of said casing section toward said sidewall and second endwall of the housing, an elongated thermally sensitive member formed solely of inelastic plastic material which is expandable as substantially a solid with substantially no change in viscosity at temperatures below 300*F. received within said casing and abutting said closed end thereof, said thermally sensitive member filling said housing and extending into said casing section, the portion of said thermally sensitive membEr within said housing being of greater cross-sectional area than the portion of said thermally sensitive member within said casing section whereby the outer surfaces of said thermally sensitive member are in contact with the inner surfaces of said casing section and housing sidewall, a piston received within the open outer end of said casing section, said piston being movable outwardly of the open outer end of said casing section into engagement with said closure member against the bias of said biasing means in response to the expansion of the thermally sensitive member at elevated temperatures, and a resilient seal interposed between said thermally sensitive member and said piston, said seal being radially expandable into tight sealing engagement with the inner surface of said casing section upon expansion of said thermally sensitive member. 